1. Field of the Invention
The present invention relates generally to Al—Zn—Mg—Cu type alloys that may possess an improved compromise between static mechanical characteristics and damage tolerance, and structural elements for aeronautical construction including partly finished strain-hardened products made from these alloys.
2. Description of Related Art
It is generally known that when manufacturing partly finished products and structural elements for aeronautical construction, certain required properties generally cannot be optimized at the same time independently of one another. When the chemical composition of the alloy or the parameters of product production processes are modified, several important properties can tend to vary in opposite directions. This is sometimes the case with respect to properties collected under the umbrella term as “static mechanical properties” (particularly the ultimate strength Rm and the yield stress Rp0.2), and second those properties known as properties relating to “damage tolerance” (particularly toughness and resistance to crack propagation). Some frequently used properties such as fatigue resistance, corrosion resistance, formability and elongation at failure are related to the mechanical properties (or “characteristics”) in a complicated and frequently unpredictable manner. Therefore, optimization of all properties of a material for aeronautical construction very often may mean making a compromise between several key parameters.
Al—Zn—Mg—Cu type alloys (belonging to the 7alloys family) are frequently used in aeronautical construction, and particularly in the construction of civil aircraft wings. For example, a sheet metal skin with a high content of 7150, 7055, 7449 alloys is often used for the extrados of wings, and stiffeners made of sections of 7150, 7055 or 7449 alloys can be used. 7150, 7050, 7349 alloys are also used for making fuselage stiffeners. The 7475 alloy is sometimes used for making wing intrados panels, particularly by machining thick plates, while extruded wing intrados stiffeners are typically made of 2xxx type alloys (for example 2024, 2224, 2027).
Some of these alloys have been known for decades, for example, the 7075 and 7175 alloys (zinc content between 5.1 and 6.1% by weight), the 7475 alloy (zinc content between 5.2 and 6.2%), the 7050 alloy (zinc content between 5.7 and 6.7%), the 7150 alloy (zinc content between 5.9 and 6.9%) and the 7049 alloy (zinc content between 7.2 and 8.2%). The compromise between toughness and yield strength is different for each of these alloys.
Patent application EP 0 257 167 A1 describes an alloy developed specifically for making hollow bodies resistant to pressure, by inverse extrusion. The composition of this alloy is as follows (in percent by weight):
Zn 6.25-8.0Mg 1.2-2.2Cu 1.7-2.8Zr 0.05Fe 0.20Fe + Si 0.40Cr 0.15-0.28Mn 0.20Ti 0.05
Values of Rm=530 MPa, Rp0.2=480 MPa, and A=15.4% cannot be exceeded for these products in a dissolved and annealed state. An increase in the content of zinc (to 8.0%), Cu (to 2.2%) and Mg (to 2.4%) causes an increase in Rm (to 570 MPa) and Rp0.2 (to 525 MPa), but these products typically have a low burst strength.
Patent application EP 0 589 807 A1 discloses a pressurized gas cylinder with a composition of Zn 6.9, Cu 2.3, Mg 1.9, Zr 0.11 that shows the following static mechanical characteristics in the L direction in the T73 temper:Rp0.2=392 MPa, Rm=459 MPa, A=15.2%.
U.S. Pat. No. 5,865,911 (Aluminum Company of America) discloses an Al—Zn—Cu—Mg type alloy with the following composition:Zn 5.9-6.7, Mg 1.6-1.86, Cu 1.8-2.4, Zr 0.08-0.15,which is taught as useful for making structural elements for aircraft. These structural elements are optimized to have high mechanical strength, toughness and fatigue strength.
Published patent application WO 02/052053 (the ‘053 application’) describes three Al—Zn—Cu—Mg type alloys with the following composition:
Zn 7.3Cu 1.6Mg 1.5Zr 0.11Zn 6.7Cu 1.9Mg 1.5Zr 0.11Zn 7.4Cu 1.9Mg 1.5Zr 0.11
The '053 application also discloses appropriate thermomechanical treatment processes for making structural elements for aircraft.
A 7040 alloy with the following normalized chemical composition is known:
Zn 5.7-6.7Mg 1.7-2.4Cu 1.5-2.3Zr 0.05-0.12Si ≦ 0.10Fe ≦ 0.13Ti ≦ 0.06Mn ≦ 0.04other elements ≦0.05 each and ≦0.15 total.
A 7085 alloy with the following standardized chemical composition is also known:
Zn 7.0-8.0Mg 1.2-1.8Cu 1.3-2.0Zr 0.08-0.15Si ≦ 0.06Fe ≦ 0.08Ti ≦ 0.06Mn ≦ 0.04Cr ≦ 0.04other elements ≦0.05 each and ≦0.15 total.
More recently, it has been observed that reducing the concentration of Cu and Mg compared with a type 7050 alloy (see EP 0 876 514 B1) may be useful. Thus, a compromise between the toughness and mechanical strength can possibly be improved for a thick plate.